Synthesis Of Novel Copolymer-Grafted Composite Thermoplastic Elastomers And Their Structure-Property Study

Thermoplastic elastomers which combine elasticity with thermoplastic properties are one of the most versatile polymers in the market and are widely used for a pyramid of important applications in our daily life like packing materials, automotive parts, adhesives, clothing, and medical devices. Most TPEs are phase-separated copolymers, in which the soft domains with a low glass transition temperature serve as matrix and the dispersed hard domains with a high glass transition temperature provide physical cross-links and endow sufficient tensile strength during deformation. In this dissertation, cellulose and rigid nanofillers were utilized to prepare a series of composite thermoplastic elastomers with strongly enhanced mechanical properties via controlled/living radical polymerization. The role of cellulose or rigid nanofillers as additional cross-linking points in the composite thermoplastic elastomers was comprehensively investigated to clarify the relationship between the micro structures and mechanical properties. The details and key conclusions are described as follows:1. Superior Thermoplastic elastomers (TPEs) are ever sought using a simple robust synthetic approach. Widely successful first-generation TPEs rely on microphase separated ABA triblock copolymers (Architecture Ⅰ). Recent multigraft copolymers represent the second-generation TPEs in which multiple branched rigid segments are dispersed in a rubbery backbone matrix (Architecture Ⅱ). Inspired by the second TPE system, we hypothesize that properties of TPEs can be achieved if we can reverse the above architecture Ⅱ design, that is, to use a rigid backbone as minority physical cross-linker and grafted chains from this backbone as a soft matrix, for which we call Architecture Ⅲ. By manipulating the compositions of random copolymers, we can access a broad spectrum of TPEs that are almost impossible to be achieved by the first two generations of architecture designs. Specifically, we use activators regenerated by electron transfer for atom transfer radical polymerization (ARGET ATRP)"grafting-from" approach to prepare a series of cellulose graft copolymers as TPEs with different mechanical properties. The obtained results indicate that the incorporation of cellulose as rigid backbone can significantly enhance the tensile strength and elastic recovery.2. Incorporation of inorganic nanoobjects into polymer matrices is an effective method for the enhancement of properties including mechanical strength, electrical conductivity, thermostability and gas permeability. The macroscopic performance of such materials is strongly related to the nanoobject size and shape, interfacial interactions, interfacial area and inter-nanofiller distance. Addition of multiwalled carbon nanotubes (MWCNTs) to TPEs via physical blending method has been widely investigated. However, little research has been performed on copolymer-grafted MWCNT composite thermoplastic elastomers (CTPEs) synthesized by directly grafting soft polymers from the surface of MWCNTs by one-pot strategy. We consider that MWCNTs can be homogeneously dispersed in the matrix and the MWCNT/polymer interface is thought to be strong through chemical bonding by graft polymerization. The key factor that governs the enhanced mechanical properties of the MWCNT-based CTPEs is an efficient load transfer between the walls of MWCNTs and polymer matrix. The microstructural evolutions during loading and unloading of the CTPEs as investigated using in-situ small-angle X-ray scattering (SAXS) indicate that the homogenously dispersed MWCNTs in the copolymer matrix are oriented gradually with their longitudinal parallel to the stretching direction as strain increases. Because of successful stress transfer from the grafted copolymer chains to the chemically bonded MWCNTs, which are orientated during tensile deformation, an apparent strain-hardening behavior can be observed for the nanocomposites. This efficient and robust synthetic approach can be further applied to the syntheses of other similar types of high performance composite thermoplastic elastomers.3. In recent years, stretchable conductive elastomers have attracted tremendous attention because they are functional materials and have many important applications in electronic devices, actuators, sensors, speakers and displayers. Composites with anisotropic nanofillers can achieve mechanical and electrical percolation at much low loadings, thus MWCNTs are considered to be ideal fillers. Copolymer-grafted MWCNT CTPEs possess strongly enhanced mechanical properties. However, these materials exhibit poor electrical conductivity due to the formation of polymer brush on the surface of MWCNT. We propose that the electrical conductivity of these copolymer-grafted MWCNT CTPEs can be significantly enhanced by incorporating minute amounts of unmodified MWCNTs, and high elastic and electrically conductive nanocomposites can be obtained by adjusting the concentration of incorporated MWCNTs. In order to reveal the relationship between the percolation threshold and micro structure of these conductive elastomers, both the rheological behavior and electrical conductivity of these comoposite TPEs were investigated. The results in this work confirm that both electrical and mechanical properties of CTPEs can be significantly enhanced by introducing unmodified MWCNTs as nanofillers to broaden the applications of such elastomeric materials. This robust and efficient strategy can be also applied to other polymer-grafted MWCNT composite materials to improve the electrical and mechanical performances.4. Magnetic copolymer-grafted nanoparticles (magnetite, FesO4) CTPEs were synthesized and characterized to generate magnetic CTPEs, which combined magnetic properties of Fe3O4nanoparticles and thermoplastic elasticity of the grafted amorphous polymer matrix. The Fe3O4nanoparticles were prepared via a hydrothermal method and were further functionalized with ATRP initiator through the condensation of silanol groups. A broad spectrum of magnetic CTPEs can be accessed by manipulating the compositions of the grafted random copolymers and the content of Fe3O4nanoparticles via this robust and effective synthetic strategy. The mechnical properties of the CTPEs were comprehensively investigated by monotonic and step cyclic tensile tests. The homogeneously dispersed Fe3O4nanoparticles acting as multiple physical cross-linking points in the CTPEs strongly enhance the mechanical properties, such as higher tensile stress and elastic recovery. An in-situ study on the microstructural evolution of the CTPEs during tensile deformation at room temperature was carried out to reveal the relationship between the micro structure and mechanical properties. The Fe3O4nanoparticles are forced to be aligned along to the tensile direction when the sample is extended, and the orientation degree increases as the strain increases, bringing out the strain-hardening hehavior. The strain-hardening can be tailored by adjusting the content of incorporated Fe3O4nanoparticles. These CTPEs are elastomeric polymer materials with strongly enhanced mechanical properties, excellent elastic recovery and adjustable magnetic performance. The present work provides a new strategy to fabricate particular functional nanomaterials with controllable properties. The magnetic composite thermoplastic elastomers might have potential applications in robotic muscles and magnetic actuators.